North Korea says the unprecedented deployment of three U.S. aircraft carrier groups "taking up a strike posture" around the Korean peninsula is making it impossible to predict when nuclear war will break out.

North Korea's U.N. Ambassador Ja Song Nam said in a letter to Secretary-General Antonio Gutteres Monday that the joint military exercises are creating "the worst ever situation" around the peninsula.

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He also said the U.S. has reactivated round-the-clock sorties with nuclear-capable B-52 strategic bombers "which existed during the Cold War times."

Ja said "the large-scale nuclear war exercises and blackmails, which the U.S. staged for a whole year without a break ... make one conclude that the option we have taken was the right one and we should go along the way to the last."